(a) Field of the Invention
The present invention relates to a retrofocus type of compact wide-angle zoom lens system consisting of a front lens group having negative refractive power and a rear lens group having positive refractive power.
(b) Description of the Prior Art
In a retrofocus type of zoom lens system, it is general to arrange an aperture stop within its rear lens group. The reason for this arrangement is to facilitate correction of distortion caused in the rear lens group by selecting the above-mentioned compositions. However, this arrangement results in displacing position of the entrance pupil backward for the front lens group, thereby enlarging the effective diameter of the frontmost lens element unavoidably and prolonging the total length of the lens system as a whole. For example, there is a conventional zoom lens system which has a sufficiently large field angle of 84.degree. at the wide position. However, this zoom lens system is not compact since the first lens element has a large effective diameter and the entire lens system has a large total length. Further, remarkable aberration remains at the wide position of this lens system.
There is another type of the conventional zoom lens system which is compact as a whole but has an unsatisfactory field angle as small as 62.degree. at its wide position. If the field angle is enlarged to about 80.degree. without changing substantially the composition of this lens system, aberrations, especially distortion will be remarkably aggravated.
It has thus been difficult for the conventional zoom lens system to satisfy all the requirements at the same time, i.e., to enlarge field angle to about 80.degree., correct aberrations favorably, design entire lens system compact as a whole and minimize effective diameter of first lens element. Therefore, the conventional wide-angle zoom lens systems, etc. have drawbacks that they are incompatible with filters having a diameter of 55 mm which are generally used with standard lens systems and must be combined with filters having a large diameter of 72 mm.